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Lubrication Advice for Forming Aluminum

Aluminum use in metalforming applications continues to rise, and not just by automotive­ parts makers. Other industries also eye the advantages in utilizing aluminum alloys. Metalformers dealing with this evolution must pay attention to numer­ous process stages, including tool design and build, and the lubrication strategy applied in the pressroom. Selecting the right lubricant for alu­minum stamping will help ensure max­imum performance in the press, as well as compatibility with the aluminum alloy at hand, and ease post-processing work such as welding. Accompany those lubricant selection criteria with best-in-class health, safety, and envi­ronmental impact and you've got a big chip in the game of stamping or draw­ing aluminum.

Lubricant Composition

For many years, straight oils, min­eral-based soluble oils or emulsions, and dry-film lubricants ruled the alu­minum-stamping market with respect to performance. They have been the standard when trying to prevent fric­tion between the aluminum workpiece and the tool steel. Straight oils, while great performers, fall behind the others when it comes to cost and secondary­process compatibility. Dry films, mean­while, require special application meth­ods, can build-up on die components and sometimes can prove difficult to remove. Therefore, soluble oils/ emulsions have become the popular choice for aluminum-stamping applications. Those oils, which contain chlorinated paraffins (CLPs), lead the way for their ability to stretch aluminum to its highest elongation point. Com­bined with good barrier and extreme­pressure (EP) protection, they allow the material to slip and flow during deep-draw processes. These compo­sitions also help reduce the impact of the aluminum-oxide layer on the surface of the substrate, which natu­rally adds some abrasiveness.

Stampers often form lighter-gauge aluminum parts using disappearing compounds as lubricants. These for­mulations find use when producing clean parts \o\There the stamper seeks to eliminate post-stamping parts clean­ing. Traditional disappearing com­pounds use petroleum distillates as a carrier for a variety of boundary film lubricants. However, in addition to flammability concerns, these petrole­um distillates evaporate into the air and create volatile organic compounds (VOCs).

New to the market, VOC-free disap­pearing compounds produce parts that can move directly to assembly or pack­aging without cleaning. For example, to manufacture the heat shield depicted in the photo on the previous page, the stamper changed from a petroleum­ distillate vanishing product to a non­hazardous type, improving employee safety and the plant environment.

Renewable-Resource-Based Oils

Vegetable and many other types of renewable-resource-based soluble oils also have begun to make a targeted impact in the aluminum-forming mar­ket. In many cases they meet or exceed the performance of mineral-based sol­uble oils, while offering a non-petro­leum-oil environmental advantage. The seed-oil base forms a strong boundary film when adsorbed onto the alu­minum surface, preventing metal pick­up and allowing good sheetmetal flow. Whether a mineral- or renewable­ resource oil-based lubricant, new chlo­rine-free soluble oils now are pushing the envelope on aluminum-stamping applications. As an example, consider that for several years, an automotive­ parts supplier formed parts from 0.055- in.-thick Type 5754 aluminum, with a 4-in. draw depth, using a chlorinated­ paraffin straight oil. By switching recently to a chlorine-free soluble oil, diluted 4:1 with water, the stamper eliminated sporadic tearing and galling while reducing lubricant use and cost. Furthermore, it eliminated all cleaning and application issues.

Now, don't count out full-synthetic chemistry just yet. While older syn­thetics containing no petroleum or vegetable oil have lagged behind sol­uble oils, many newer products have risen to the occasion. Packed with newer polymer- and ester-additive technology, today's full synthetics also prevent the pickup and drag normally experienced when forming aluminum.

Corrosion

As with most other substrates, com­patibility with the stamping lubricant is of great concern with aluminum, especially when diluting the lubricant with water. Although ferrous staining and corrosion aren't of concern here, metal formers will prefer to avoid any staining or pitting beyond the normal aluminum-oxide layer buildup.

Different alloys and finishes come with varying built-in corrosion resistance. For example, Types 5052 and 6061 aluminum offer excellent corrosion resistance and find use due to their good formability. How­ever, higher-strength alloys such as Types 7075 and 2024 offer average or poor corrosion resistance.

When dealing with different alloys, certain lubricants are naturally passive while other lubricants must be buffered for pH and include additional corrosion inhibitors to prevent excessive attack on the substrate surface. Straight oils can offer very good corrosion inhibition due to the absence of water. Dry films also perform well, once any water car­rier has evaporated. The extent of cor­rosion from water-soluble lubricants can vary depending on the pl-I of the water and the lubricant, as well as water quality and the additives in the product. Higher alkalinity can destroy the aluminum-oxide protective layer, which leads most water-soluble prod­ucts to require corrosion inhibitors for proper protection.

o minimize the propensity for staining or corrosion in metalforming aluminum, metalformers should per­form quick stain tests on the substrates. To carry out these tests, mix the water­soluble lubricant to the desired con­centration, then cut an aluminum coupon and submerse it half-way into the diluted lubricant. After 24 hr. soak­ing at room temperature, inspect the coupon for staining and discoloration.

Post Processing

After forming, the presence of lubricant residue on the stamped aluminum parts can impact secondary processes such as cleaning, welding and painting. Depending on the severity of the form­ing, the remaining residue may be heavy or light, oily or sticky, thick or thin, and possibly pooled in corners or all over the part. Whatever the state, the stamper may have to plan for sec­ondary processing to remove the residue.

In most cases with straight oils and dry films, the job calls for a solvent or high-alkaline water-based cleaner to remove the residue, as even the slight­est amount of residue can impede welding and painting. For difficult forming operations requiring a straight oil, the stamper can opt for a lubricant with a special additive package to allow for quicker, simpler cleaning in an alka­line parts washer run at a relatively low temperature.

Soluble oils and synthetics with higher dHution ratios can leave mini­mal and dry-to-the-touch residues that, in many cases, can be welded through, depending on the composition and dilution required for forming. However, most residues, even if heavy, will wash off in a mild-alkaline cleaner or water rinse.

Regardless of the cleaning method selected, stampers must exercise cau­tion. Perform tests on parts to ensure that the cleaning solution does not cause an adverse reaction, such as etching or staining of the workpiece material.

Health, Safety and Environmental

Aluminum use continues to expand, not just for its favorable strength-to­weight ratio, but also for its environ­mental advantages. So, why not apply this philosophy when selecting the optimal lubricant?We should advance beyond just looking for the best form­ing lubricant that performs well during secondary processing. Newer lubri­cants also can improve operator health and safety, and positively impact the environment.

EP agents continue to find use in fluids to help with carrying the friction load ben.veen the aluminum \.vorkpiece and the tooling materials. CLPs have been the talk of the industry lately, with the recent push by the EPA to ban medium- and long-chain CLPs based on data suggesting high bio-accumu­lation and persistency rates. While this debate continues, lubricant manufacturers have begun to look at EPA­approved and registered very-long­chain CLPs to diminish environmental bio-accumulation and persistency rates.

Other robust additives that reduce the reliance on traditional EP agents also have made headway into modern formulations, creating more eco­friendly stamping lubricants-not just environmentally friendly, but also friendly to users. With the GHS system in place now for a few years, safety datasheets reveal the potential hazards of a product with written warnings and pictograms.

A quick visual can tel1 a stamper how each lubricant stacks up and allow managers to make informed decisions regarding health and safety. If your lubricant now comes packaged with any pictograms, ask yourself if you would use the product in your personal life. If not, seek a better option that can significantly reduce your health, safety and environmental footprint.